The development of a high data rate, free space optical communication (FSOC) system has some limitations related to atmospheric turbulence. Laser beams experience three major effects under turbulence. First, the beam phase front is distorted by fluctuations in the refractive index, which causes intensity fluctuations that are known in the art as scintillations. The scintillations are the most severe problem and result in a significant increase of the bit error rate (BER) and degradation of laser communication system performance. Second, eddies having a size greater than the beam diameter randomly deflect the laser beam as a whole. This phenomenon is called “wandering”. Third, propagating a laser beam through a turbulent atmosphere causes the laser beam to spread more than what is predicted by diffraction theory. For example, a gigabit data rate communication channel can operate with BER of 10−9  over a distance not more than about 2.5 kilometers (km), even for clear weather. New approaches are needed to overcome this limitation.
Several approaches have been developed to mitigate the effects of turbulence on laser communication system performance. Some of these approaches are concerned with aperture averaging, phase diffusers, adaptive optics, and special data communication encoding (for a review, see: Andrews et al. in “Laser Beam Scintillation with Applications, SPIE Press, Bellingham, Wash., USA, 2001). None of these approaches eliminates the negative influence of turbulence on laser communication completely. New approaches are needed to avoid the negative influence of the atmospheric turbulence.
Recently, a technique of scintillation reduction based on the utilization of partially coherent beams (i.e., beams with multiple coherent spots in their transverse section) was demonstrated. Combining partially coherent beams with a time-averaging photodetector leads to a significant scintillation reduction with the corresponding improvement of the BER by several orders of magnitude. This phenomenon, however, cannot be utilized for a conventional encoding scheme in which the information is encoded in the form of a series of pulses. The main limitation of this technique is related to the requirement that the correlation time between different spatially coherent spots be shorter than the response time of the photodetector. This means that the spatial light modulator (SLM) must have an operating frequency ν higher than the bandwidth of the photodetector, corresponding to its inverse response time ν>>T−1. Since the photodetector bandwidth must be higher than the data rate of the communication channel νCOM, T−1>>νCOM, the highest data rate is limited by the highest frequency of the SLM ν>>νCOM. To date, the highest frequency SLMs based on multiple quantum wells (MQW) can only operate at frequencies up to tens of MHz.
In the case of a non-coherent, wideband source such as a light emitting diode (LED), the coherence time could be shorter than the time response of a photodetector. It appears, however, that an LED has not been used for gigabit rate communication because of its limited modulation rate of less than a few hundred megahertz (MHz).
There remains a need for a high data rate, free space optical communication system that suppresses the negative influence of atmospheric turbulence.